Field of the Invention
The present invention relates to an optical waveguide device that enables high-speed refractive index modulation with low optical loss and low driving voltage and a method of manufacturing the optical waveguide device.
Description of the Related Art
In recent years, the application of small-footprint optical integrated circuits such as planar optical waveguides to optical-fiber communication devices, in particularly, to optical transport equipment used for long-haul or metro-area wavelength-division multiplexing optical-fiber communication and optical interconnect equipment used in data centers and the like has been studied.
U.S. Pat. No. 7,085,443 (hereinafter referred to as PTL 1) discloses a single-mode silicon rib waveguide, in which refractive index is controlled by changing carrier density in PN junction formed in the transverse direction and the position of the PN junction boundary changes in a corrugated form along a direction of light propagation in the core of the optical waveguide.
Zhi-Yong Li, Dan-Xia Xu, W. Ross McKinnon, Siegfried Janz, Jens H. Schmid, Pavel Cheben and Jin-Zhong Yu, “Silicon waveguide modulator based on carrier depletion in periodically interleaved PN junctions”, Optics Express vol. 17, no. 18, pp. 15947-15958 (2009) (hereinafter referred to as NPL 1) discloses technical information about the optical characteristics of a silicon waveguide in a configuration similar to that of the silicon rib waveguide of PTL 1. NPL 1 reports that, in the configuration disclosed in PTL 1 and in an optical modulator consisting of a silicon optical waveguide having a similar configuration to the above-described configuration, Bragg reflection caused by the periodic profile of the refractive index is negligible. Furthermore, NPL 1 discloses design exemplification of an optical modulator operating at a longer wavelength than the Bragg wavelength.
United States Patent Application, Publication No. 2012/0189239 (hereinafter referred to as PTL 2) discloses an optical modulator in which optical loss is reduced by using a configuration in which the depletion region in the PN junction is sandwiched between the first and second intrinsic regions. The first and second intrinsic regions are disposed contiguously to respective one of the two opposing side walls of the optical waveguide.
In the silicon rib waveguide disclosed in PTL 1 in which the position of the PN junction boundary changes in a corrugated line along a direction of the light propagation in the core of the optical waveguide, since the effective length of the PN junction increases, the driving voltage is reduced. However, in this case, there is a problem of an increase in optical loss due to an increase of optical absorption by carriers. In addition, there is another problem that it is not possible to reduce parasitic capacitance caused by the fringe electric fields from slab regions existing in side sections of the rib waveguide and high-speed operation is impaired. Therefore, in a case in which the above-described silicon rib waveguide is used, high-speed refractive index modulation with low optical loss is difficult.
In a design based on NPL 1, when a rib waveguide having a corrugated distribution profile in high refractive index contrast is used, return loss is decreased due to Bragg reflection and optical feedback to a laser light source generating incident light to the optical modulator is significant and mode hopping is generated, whereby the laser light source becomes unstable. As a result, there is a problem that it is not possible to generate stabilized optical modulation signals.
In the optical modulator disclosed in PTL 2, when the driving voltage is reduced, the refractive index modulation degrades. Therefore, there is a problem that it is not possible to reduce the driving voltage. Furthermore, in PTL 2, there are additional problems that the influence of fabrication error is significant and it is not possible to provide an optical modulator with small quality variation.
As described above, an object of the invention is to realize an optical waveguide device that is suitable for small-footprint optical integrated circuits such as an optical modulator enabling high-speed refractive index modulation with low optical loss and low driving voltage and allows only small quality variations for application to optical-fiber communication devices, in particularly, to optical transport equipment used for long-haul or metro-area wavelength-division multiplexing optical-fiber communication and optical-interconnect equipment used in data centers and the like.